You can browse lot of books for your references from the following websites.

1.www.abebooks.com
2.www.bizrate.com
3.www.alibris.com
4.www.bartleby.com
5.www.powells.com
6.www.oreilly.com
7.www.books.com
8.www.scholastic.com
9.www.hoganbooks.com
10.www.netlibrary.com
11.www.ipl.org
12.www.hungryminds.com
13.www.booksamillion.com
14.www.varsitybooks.com
15.www.robotbooks.com
16.www.wiredforbooks.org
17.www.wordsworth.com
18.www.ala.org
19.www.macmillan.com
20.www.bol.com